The present disclosure relates to a photovoltaic device and photovoltaic device manufacturing. More particularly, the present disclosure provides a photovoltaic device with an aluminum electroplated back side surface field and a method of forming the same.
A photovoltaic device is a device that converts the energy of incident photons to electromotive force (e.m.f.). Typical photovoltaic devices include solar cells, which are configured to convert the energy in the electromagnetic radiation from the Sun to electric energy. Each photon has an energy given by the formula E=hv, in which the energy E is equal to the product of the Plank constant h and the frequency v of the electromagnetic radiation associated with the photon.
A photon having energy greater than the electron binding energy of a matter can interact with the matter and free an electron from the matter. While the probability of interaction of each photon with each atom is probabilistic, a structure can be built with a sufficient thickness to cause interaction of photons with the structure with high probability. When an electron is knocked off an atom by a photon, the energy of the photon is converted to electrostatic energy and kinetic energy of the electron, the atom, and/or the crystal lattice including the atom. The electron does not need to have sufficient energy to escape the ionized atom. In the case of a material having a band structure, the electron can merely make a transition to a different band in order to absorb the energy from the photon.
The positive charge of the ionized atom can remain localized on the ionized atom, or can be shared in the lattice including the atom. When the positive charge is shared by the entire lattice, thereby becoming a non-localized charge, this charge is described as a hole in a valence band of the lattice including the atom. Likewise, the electron can be non-localized and shared by all atoms in the lattice. This situation occurs in a semiconductor material, and is referred to as photogeneration of an electron-hole pair. The formation of electron-hole pairs and the efficiency of photogeneration depend on the band structure of the irradiated material and the energy of the photon. In case the irradiated material is a semiconductor material, photogeneration occurs when the energy of a photon exceeds the band gap energy, i.e., the energy difference of the conduction band and valence band.
The direction of travel of charged particles, i.e., the electrons and holes, in an irradiated material is sufficiently random (known as carrier “diffusion”). Thus, in the absence of an electric field, photogeneration of electron-hole pairs merely results in heating of the irradiated material. However, an electric field can break the spatial direction of the travel of the charged particles to harness the electrons and holes formed by photogeneration.
One exemplary method of providing an electric field is to form a p-n or p-i-n junction around the irradiated material. Due to the higher potential energy of electrons (corresponding to the lower potential energy of holes) in the p-doped material with respect to the n-doped material, electrons and holes generated in the vicinity of the p-n junction will drift to the n-doped and p-doped region, respectively. Thus, the electron-hole pairs are collected systematically to provide positive charges at the p-doped region and negative charges at the n-doped region. The p-n or p-i-n junction forms the core of this type of photovoltaic device, which provides electromotive force that can power a device connected to the positive node at the p-doped region and the negative node at the n-doped region.
The photon generated electrons and holes are to be collected at the n-type and p-type semiconductor materials, respectively. However, the number of carriers can decrease significantly because they recombine with each other before they get collected. This recombination becomes more pronounced when there are more recombination centers, i.e., locations where the recombination is more likely to occur. One of main recombination is surface recombination, which occurs at the semiconductor surface.
In order to improve the performance of a solar cell, surface recombination needs to be suppressed. One way of doing that is to prevent the carrier to reach the surface. For example, aluminum was used in silicon solar cells to form a highly doped p-type Si layer at the surface of a p-type Si substrate. This highly doped p-type Si layer creates an electrical field, commonly called a back surface field (BSF), which repels the electrons at the vicinity of this layer, and thus limits the surface recombination. A better way of suppressing surface recombination is to use a layer of material to change the semiconductor surface behavior, so called passivation, and to suppress the recombination even if the carriers reach the surface. A commonly used example is silicon nitride on Si solar cells.
The majority of solar cells currently in production are based on silicon wafers. The surface recombination on the n-type Si was improved by using a silicon nitride passivation layer, and on the p-type Si by using a screen printed blanket film of aluminum to form the BSF. In addition, the printed aluminum also acts as the electrical contact to the p-type Si. The electrical contact to the n-type Si was formed through the nitride passivation layer by methods such as screen printing Ag.
Screen printing is attractive due to its simplicity in processing and high throughput capability; however, the high contact resistance, high paste cost, shadowing from wide conductive lines, high temperature processing, and mechanical yield loss are disadvantages that have not been overcome even after thirty plus years of research and development.
Furthermore, because a nitride passivation layer better suppresses the surface recombination than a BSF, a local aluminum BSF contact with most of Si surface passivated by silicon nitride is needed. However, direct screen printing of patterned aluminum through silicon nitride is not available. Local deposition of aluminum directly on p-type Si and the local formation of BSF in a silicon nitride pattern are highly desired.
For advanced and experimental high efficiency solar cells, vacuum based metallization processes are used to avoid the disadvantages of screen printing. The high cost and low throughput of vacuum processes prohibit the implementation of these processes in single emitter solar cells, the majority in the current photovoltaic industry.